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OSIRIS-REx spacecraft zooms by Earth on its way to an asteroid

Set course for Bennu

NASA's Goddard Space Flight Center Conceptual Image Lab

By Leah Crane

To Bennu and back again! At around 1700 GMT today, NASA’s OSIRIS-REx spacecraft will hurtle around Earth at 8.5 kilometres per second, giving it the boost it needs to reach the asteroid Bennu next August.

Since its 2016 launch, the spacecraft has been circling the sun in Earth’s orbit, just ahead of the planet. Bennu also revolves around the sun, but its orbit is slightly tilted compared to Earth’s, so it only crosses our path at two points during the year. OSIRIS-REx will therefore have to make an adjustment in its path to intersect with that of the asteroid.

To do that, the spacecraft will slingshot around Earth, passing over Australia before making its closest approach to the planet, at about 17,000 kilometres above Antarctica. This will tilt its orbit by about 6 degrees – enough to align it with Bennu.

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As OSIRIS-REx passes over Earth, many of its instruments will turn on, including cameras that will take pictures of Earth and the moon. These pictures will be used to calibrate the cameras and test procedures for processing data, in preparation for more intensive data collection once the spacecraft nears its destination.

“It’s kind of a test run,” says Daniella DellaGuistina, an OSIRIS-REx team member at the University of Arizona in Tucson. “It’ll be good to find kinks in our process before we actually arrive at the asteroid.”

Onward to Bennu

After the spacecraft hurtles by, it will keep cruising for another year, travelling more than 1 billion kilometres before it reaches Bennu. “It’s smooth sailing from here on out, give or take some small adjustments,” says Cristina Thomas, an OSIRIS-REx team member at the Planetary Science Institute in Tucson.

Once OSIRIS-REx reaches Bennu, it will take a sample from the asteroid’s rocky surface and will return to Earth in 2023. If successful, the mission will mark the first time the US has ever brought samples from an asteroid back to Earth, and the second time any dust has been returned from an asteroid.

The spacecraft won’t land on the asteroid, but will get close enough to kick up some dust. When it gets within a few metres of the surface, OSIRIS-REx will extend a mechanical arm and blow out a puff of nitrogen gas, which will cause a small plume of dust particles to stream up from Bennu.

The asteroid is only about 500 metres wide and researchers are unsure of its exact mass, so navigating OSIRIS-REx into orbit around the tiny asteroid will be complicated – not to mention the long voyage home.